The present invention relates to a method and apparatus for estimating the state of charge of a rechargeable battery.
A rechargeable battery as a power source for a motor and as a drive source for various types of loads is applicable for use in a pure electric vehicle (PEV), a hybrid electric vehicle (HEV), and the like.
In a hybrid electric vehicle (HEV), when the engine output is more than necessary for driving the vehicle, surplus power is used to drive a generator and charge a rechargeable battery (e.g., nickel-metal hydride (NiMH) battery). On the other hand, when the engine output is low, the electric power of the rechargeable battery is used to drive a motor and assist the engine. In such a case, the rechargeable battery is discharged. Accordingly, in an HEV, the charging and discharging of the rechargeable battery must be controlled to maintain the rechargeable battery in a proper operational state.
Japanese Laid-Open Patent Publication No. 2007-292648 describes a controller that optimizes the fuel consumption efficiency of an HEV by estimating the state of charge (hereinafter referred to as the “SOC”) of a rechargeable battery with a control program using detection data related to the rechargeable battery such as the charge-discharge current I, the terminal voltage V, and the temperature T. The controller controls the SOC so as to optimally balance power assist, which is performed when accelerating the vehicle by driving the motor, and energy recouping (regenerative braking), which is performed when decelerating the vehicle.
SOC control is executed to adjust the charging and discharging of a rechargeable battery so that the SOC is in a range of, for example, 50% to 60%. More specifically, charging is performed when the SOC becomes 50% or less, and discharging is performed when the SOC becomes 60% or greater.
For accurate execution of the SOC control the SOC is estimated in the following manner in the prior art.
First, data sets for the terminal voltage V and the charge-discharge current are obtained during a predetermined period Δt (e.g., 60 sec) and stored in a memory. Then, the data sets are used to obtain a primary approximate line (approximate line of the voltage V and current I) by performing statistic processing using the least squares approach. A V intercept of the V-I approximate line is obtained as a non-load voltage Vo.
Next, the polarization voltage Vp at the rechargeable battery is calculated. The calculation may be performed through one of the following two processes. In the first process, a cumulative capacitance Q (i.e., Q=∫I) is obtained by cumulating the charge-discharge current I over a predetermined period. Then, the polarization voltage Vp is calculated from a variation amount ΔQ of the cumulative capacitance Q (i.e., the difference between the cumulative capacitance Q obtained during the previous predetermined period and the cumulative capacitance Q obtained during the present predetermined period) and the temperature T of the rechargeable battery (e.g., −30° C.≦T≦60° C.). In the second process, an attenuation amount and generation amount ΔVp of the polarization voltage Vp are calculated with a predetermined calculation expression. Then, the attenuation amount and the generation amount ΔVp are used to calculate the polarization voltage Vp. The polarization voltage Vp is the difference between the theoretical open circuit voltage (OCV) of the rechargeable battery that is determined by the electromotive force Ve and the actual open circuit voltage of the rechargeable battery.
Subsequently, the electromotive force Ve is obtained by subtracting the polarization voltage Vp from the non-load voltage Vo. Then, the SOC is estimated from the electromotive force Ve by referring to a Ve-SOC characteristics table, which is prepared beforehand.
In the prior art SOC control executed for an HEV, the polarization voltage Vp is estimated when the HEV is activated (i.e., when turning on an ignition switch). During the period from when the HEV is activated to when the engine is started, the rechargeable battery is assumed to be in a charge-discharged state (hereinafter referred to as the “battery-current state”). Accordingly, in the prior art, the polarization voltage Vp is calculated under the assumption that the rechargeable battery is in the battery-current state. Japanese Laid-Open Patent Publication No. 2003-197275 describes such a prior art approach for calculating the polarization voltage Vp.
However, the inventors of the present invention have found that the rechargeable battery is continuously in a no-battery-current state, in which the rechargeable battery does not perform charging or discharging, over a rather long time when the crankshaft is not rotating such as during the period from when the HEV is activated to when the engine is started. The inventors have also found that such a no-battery-current state continues over a rather long time from when the engine is stopped after parking the HEV, which has been driven.
In the battery-current state, the charging and discharging of the rechargeable battery is repeated in a complicated manner. This results in rapid loss of ionic species, which are the cause of the polarization voltage Vp, on an electrode reactive interface and attenuates the polarization voltage Vp at a high speed. In the no-battery-current state, current does not flow to the rechargeable battery. Thus, the ions species on the electrode reactive interface are slowly lost. For this reason, in comparison to the battery-current state, the polarization voltage Vp is attenuated at a significantly lower speed.
Therefore, if the calculation approach described above (i.e., calculation approach assuming the battery-current state) is used when the polarization voltage Vp is in an attenuated state, the polarization voltage Vo is attenuated more than actual. As a result, the calculated polarization voltage Vp, which deviates from the actual state, produces errors in the electromotive force Ve and the SOC estimated value, which are calculated from the polarization voltage Vp.